Since the mid 1980's, efforts have been made to improve the properties, for example, wear resistance and hence the performance of tool coatings. At that time, the common practice was to coat cutting tools with TiN. However, due to its relatively poor oxidation resistance at elevated temperatures, alloying Al in (Ti,Al)N was suggested and implemented with good results in the mid-1980's.
Today (Ti,Al)N based coatings are among the most common hard and protective coating materials used in metal cutting applications. The cubic, B1, structure of (Ti,Al)N, as a monolith layer or part of a laminated coating structure, combine attractive mechanical properties such as high hardness and improved temperature and oxidation resistance providing good performance in metal machining applications. The technological benefits of (Ti,Al)N and its excellent physical properties, especially at elevated temperatures, is partly explained in terms of a spinodal decomposition process during which cubic (Ti,Al)N decompose isostructurally into coherent cubic c-AlN- and c-TiN-enriched domains. The combination of elastic properties and a lattice mismatch between coherent c-AlN- and c-TiN-enriched domains leads to significant age hardening during which the hardness of (Ti,Al)N thin layers have shown to increase with between 15% and 20%. At further aging, c-AlN transforms into the thermodynamically stable hexagonal, wurtzite B4 structure, h-AlN resulting in a dual phase structure comprising c-TiN and h-AlN with reduced mechanical properties.
To further enhance the performance of tool coatings, a wide range of ternary and quaternary systems have been investigated. For example, it has been shown that alloying Cr in (Ti,Al,Cr)N improves the coating behavior in metal cutting applications. International published patent application WO2012/069475 discloses a coated cutting tool having a hard and wear resistant coating comprising a layer of (TicAlaCrbMed)(CzOyNx) where 0.10<a<0.60, b+d>0.20, c>0.05, 0≤d<0.25, 0.75<x<1.05, 0≤y<0.25 and 0≤z≤0.25 with a layer thickness between 0.5 and 10 μm.
Todays industry continuously seeks solutions for economic and high productivity/feed-through manufacturing. To meet these demands there is a need for new materials with advanced properties to improve tool life during operation. Within the metal cutting tool industry, a major part of this effort is focused to improve the wear behavior of the cutting tools by designing the properties of the coating material used in the application. Typically, a high productivity/feed-through cutting process results in a dramatic increase of the tool temperature and hence a coating material with a high temperature wear resistance is essential.